+=head1 RETURN VALUES
+
+DES_set_key(), DES_key_sched(), DES_set_key_checked() and DES_is_weak_key()
+return 0 on success or negative values on error.
+
+DES_cbc_cksum() and DES_quad_cksum() return 4-byte integer representing the
+last 4 bytes of the checksum of the input.
+
+DES_fcrypt() returns a pointer to the caller-provided buffer and DES_crypt() -
+to a static buffer on success; otherwise they return NULL.
+